U.S. patent number 6,142,458 [Application Number 09/182,747] was granted by the patent office on 2000-11-07 for mixing system for dispersion of gas into liquid media.
This patent grant is currently assigned to General Signal Corporation. Invention is credited to Richard A Howk.
United States Patent |
6,142,458 |
Howk |
November 7, 2000 |
Mixing system for dispersion of gas into liquid media
Abstract
In order to sparge gas into a liquid or liquid suspension in a
tank wherein a principally axial flow pattern downwardly towards
the bottom of the tank and then upwardly along the side wall of the
tank returning axially downward is established by an axial flow
impeller, a disc of a diameter less than the diameter of the
impeller is spaced axially therefrom in the direction of the outlet
flow towards the bottom of the tank from the impeller so as to turn
the axial flow, radially, thereby establishing a pressure gradient
which prevents the collection of gas released by a sparge between
the disc and the bottom of the tank and flooding of the impeller.
The gas is released in the axial flow from the tip region of the
impeller thereby facilitating the shearing of the gas into fine
bubbles promoting mass transfer of the gaseous phase into the
liquid phase in the tank. Since flooding is inhibited, as much as
six times the volume of gas (gas rate) can be handled as may be the
case without the disc.
Inventors: |
Howk; Richard A (Pittsford,
NY) |
Assignee: |
General Signal Corporation
(Rochester, NY)
|
Family
ID: |
22669846 |
Appl.
No.: |
09/182,747 |
Filed: |
October 29, 1998 |
Current U.S.
Class: |
261/93; 261/123;
261/124 |
Current CPC
Class: |
B01F
23/233 (20220101); B01F 27/91 (20220101); B01F
27/1155 (20220101); B01F 27/113 (20220101); B01F
23/23364 (20220101); B01F 27/191 (20220101); B01F
27/115 (20220101); B01F 23/23362 (20220101); B01F
23/23314 (20220101) |
Current International
Class: |
B01F
7/22 (20060101); B01F 7/16 (20060101); B01F
3/04 (20060101); B01F 7/00 (20060101); B01F
003/04 () |
Field of
Search: |
;261/93,121.1,123,124 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Simmons; David A.
Assistant Examiner: Hopkins; Robert A.
Attorney, Agent or Firm: Lukacher; Martin Lukacher; Kenneth
J.
Claims
What is claimed is:
1. A mixing system for dispersing a gas into a liquid which
comprises an axial flow impeller having blades rotatable about an
axis to provide said flow principally along said axis, a member
presenting a surface opposing the axial flow from said impeller,
said surface extending radially from said impeller and being spaced
axially away from said impeller, a sparge releasing gas into a
region radially outward of said member in the axial flow from said
impeller, said spacing from and said radial extent of said surface
with respect to said impeller being sufficiently proximate to said
blades across a sufficient radial extent of said blades to turn the
axial flow of said impeller into radial flow across said blades of
a velocity sufficient to inhibit the collection of gas on said
impeller and the flooding thereof by said gas.
2. The system according to claim 1 wherein said spacing is from 5%
to 20% of the impeller diameter and said surface has a periphery
spaced inwardly from the path circumscribed by the tips of the
blades of said impeller as it rotates.
3. A mixing system for dispersing a gas into a liquid which
comprises an axial flow impeller having blades rotatable about an
axis to provide said flow principally along said axis, a member
presenting a surface opposing the axial flow of said impeller, said
surface extending radially of said impeller and being spaced
axially away from said impeller and inwardly from the tips of said
blades, a sparge releasing gas into a region radially outwardly of
said member, and said radial extent of said surface with respect to
said impeller and the proximity of said surface to said blades
being sufficient to turn the axial flow of said impeller into
radial flow of a velocity sufficient to inhibit the collection of
gas on said impeller and the flooding thereof by said gas, and
wherein said impeller has a diameter measured across the path of
the tips of the blades thereof as said impeller rotates, said axial
spacing of said surface from said impeller measures from the
midline of said impeller being in the range of about 5% to 10% of
the diameter of the impeller and the radial extent of said surface
being about 50% to 85% of the diameter of said impeller.
4. A system according to claim 3 wherein said axial spacing of said
surface is 7.5% of the diameter of said impeller and said radial
extent of said surface is approximately 75% of the diameter of said
impeller.
5. The system according to claim 3 wherein said member is a disc
co-axial with said impeller.
6. The system according to claim 5 wherein said disc is rotatable
with said impeller and mounted on the same shaft as said
impeller.
7. The system according to claim 5 wherein said disc is mounted
stationary with respect to said impeller.
8. The system according to claim 3 wherein said region is defined
by the periphery of said surface which is disposed radially inward
of the path circumscribed by said tips.
9. The system according to claim 3 wherein said member is a disc
co-axial with said impeller and said sparge is a tubular ring
spaced axially from said impeller further away than said disc in
the direction of axial flow from said impeller, said ring having a
plurality of holes along the periphery thereof, said ring, where
said holes are located, being of a larger diameter than said disc
and a smaller diameter than said impeller.
10. The system according to claim 3 wherein said member is a first
disc co-axial with said impeller and said sparge comprises a second
disc co-axial with said first disc and of about the same diameter
as said first disc, said discs defining a space having an inlet for
said gas and an outlet for said gas, said outlet being around the
periphery of said space.
11. The system according to claim 3 further comprising a tank
having a bottom and side walls, said tank containing said liquid to
a level above the bottom of said tank in which said impeller,
member and sparge are submerged, said impeller providing an axial
flow pattern having principally axial flow components downwardly
from said impeller towards the bottom of said tank and upwardly
along the walls of said tank, said impeller being spaced from said
bottom of said tank a distance measured from the midline of said
impeller equal about to or less than the diameter of said
impeller.
12. The system according to claim 11 wherein said spacing of said
impeller to the bottom of said tank is about 2/3 of the diameter of
said impeller.
13. The system according to claim 11 wherein the ratio of the
diameter of said impeller to the diameter of the tank is about 1/3.
Description
DESCRIPTION
The present invention relates to mixing systems and particularly to
mixing systems which disperse or sparge gas or other fluids into a
liquid (by which term is meant to include liquid suspensions) and
which enhance gas handling capacity. The systems provided by the
invention are useful especially for mass-transfer of the gas or
other fluids into a liquid phase in a tank to promote chemical
reactions or other processes which rely upon the introduction and
blending of gas or other fluids with the liquid in the tank.
The gas handling capability of mixing systems using axial flow
impellers is limited by the tendency for such impellers to flood.
Upon flooding, the gas flow predominates over the pumping action of
the impeller, and the rate at which gas is introduced has been
limited to avoid flooding. In U.S. Pat. No. 4,882,098 issued Nov.
21, 1989 to Ronald J. Weetman, a mixing system for sparging gas in
a tank, where circulation is established by an axial flow impeller,
is disclosed. In the Weetman system the sparge is located in a
manner to reduce the effects of flooding and increase the gas rate
which can be handled. Other systems using axial flow impellers have
not attacked the flooding problem directly, but have proposed
various geometrical relationships between the impeller and the
sparge. See Davis, U.S. Pat. No. 4,660,989, Apr. 28, 1997; and Di
Gregorio, U.S. Pat. No. 3,814,986, Jun. 4, 1974. Other proposals
have included baffles for distributing the gas from a sparge
device. See Fulweiler, U.S. Pat. No. 1,632,758, Jun. 14, 1927;
Hise, U.S. Pat. No. 4,228,112, Oct. 14, 1980, Takeuchi, U.S. Pat.
No. 4,519,959, May 28, 1985; and Koslow, U.S. Pat. No. 4,643,852,
Feb. 17, 1987.
It is a feature of the invention to provide a mixing system whereby
gas handling capabilities are enhanced by hydraulically inhibiting
the collection of gas on the axial flow impeller and redirecting a
portion of the axial flow discharge to effectively inhibit and
forestalling flooding. The hydraulic means provided by the
invention may be implemented with a simple disc in such geometrical
relationship with the impeller and the sparge that the flow
produced by the impeller creates a pressure differential which
prevents the gas stalling the tips of the impellers which prevents
cavitation, but without, as might otherwise be expected with a disc
in close proximity to the impeller, interfering with the axial flow
from the impeller into which the gas is released from the
sparge.
Accordingly, it is the principal object of the invention to provide
an improved mixing system enabling and increase in gas volume or
gas rate being dispersed into a liquid circulated by an axial flow
impeller by inhibiting flooding of the impeller.
It is another object of the present invention to provide an
improved mixing system for gas dispersion in liquids (which term
liquid includes liquid suspensions).
It is another object of the invention to provide an improved mixing
system for sparging gas into circulating liquid produced by an
axial flow impeller, as exhibited by maintenance of the K factor
(ratio of power used by the mixing impeller at constant speed of
rotation, while gas is being disbersed to the power used without
gas dispersion) with increasing gas rate.
It is a still further object of the present invention to provide an
improved gas dispersion mixing system having a gas delivery
mechanism (a sparge) which is less prone to clogging than pipe or
ring sparges which have conventionally been used for the
purpose.
It is a still further object of the present invention providing an
improved mixing system for gas/liquid operation in dispersing gas
into liquid for mass transfer of the gaseous phase into the liquid
phase, which mixing system uses conventional axial flow impellers,
thereby simplifying mixing system design because of the efficiency
of such impellers (power number) and of the weight thereof, which
affects sizing of shafts and other components of the impeller
drive.
It is of further object of the present invention to provide an
improved mixing system for enhanced gas dispersion or sparging,
which system is simple in design requiring only a flat disc
co-axial with an axial flow impeller and of sufficient diameter
relative to the impeller diameter and in sufficiently close
proximity to the impeller to turn axial flow into radial flow which
sweeps across the impeller and inhibits the collection of gas on
the impeller blades and consequential flooding of the impeller, and
also without interference with the dispersion of gas into the
liquid circulated by the impeller.
Briefly described, a mixing system for dispersing a gas into a
liquid according to this invention utilizes an axial flow impeller
rotatable about an axis to provide flow principally along the axis.
Hydraulic means are provided for inhibiting flooding of the
impeller by the gas and includes a member presenting a surface
opposing the axial flow from the impeller and extending radially of
the impeller. This surface of this disc is spaced axially away
from, but in proximity to the impeller. A sparge releases the gas
into the axial flow from the impeller in a region radially outward
of the member. The spacing from and radial extent of the surface
with respect to the impeller is sufficient to turn the axial flow
from the impeller into radial flow over a substantial radial extent
of the impeller, up to but not including the tips of the impeller.
This radial flow is of velocity sufficient to inhibit the
collection of the gas on the impeller and the flooding thereof by
the gas.
The foregoing and other objects, features, and advantages of the
invention, as well as presently preferred embodiments thereof, will
become more apparent from a reading of the following description in
connection with the accompanying drawings wherein:
FIG. 1 is a view in elevation and in a vertical section through a
tank along a diameter of the tank, which illustrates a mixing
system embodying the inventions;
FIG. 2 is a sectional plan view, taken along the line 2--2 in FIG.
1;
FIG. 3 is an enlarged elevational view showing the impeller, the
disc member and a sparge ring and the geometrical relationship
thereof which enhances gas handling capability in accordance with
the invention;
FIG. 4 is a view similar to FIG. 1 of a gas dispersion mixing
system in accordance with another embodiment of the invention;
FIG. 5 is a sectional view taken along the line 5--5 in FIG. 4;
FIG. 6 is a fragmentary and enlarged view of the mixing system
shown in FIG. 4 and illustrating the flow and gas dispersion
patterns which are believed to occur in the operation of the
system;
FIG. 7 is a pair of curves showing the K factor without respect to
aeration of or gasification number which represents the volume of
gas introduced for a volume of liquid being pumped or circulated
for a system shown in FIGS. 1, 2 and 3 without the disc in dash
lines and with the disc in solid lines, thereby showing the
improvement in gas handling capacity afforded by the introduction
of the disc.
Referring to FIGS. 1, 2, and 3 there shown a tank 10 having a
bottom 12 and a sidewall 14. The tank is filled with liquid which
is circulated by a conventional axial flow impeller 16. The
impeller 16 has three blades 18 mounted on a hub, 20. The hub is
attached to a shaft 22 of a drive system including a motor and
gearbox 24 resting on a beam 26 extending over the top of the tank
10. The shaft rotates about a vertical axis, driving the impeller
16 to circulate the liquid in the tank principally in an axial
direction. In this embodiment, the impeller 16 is down pumping
toward the bottom of the tank, such that the axial flow is
downward. The flow is turned radially outward from the shaft 22
(See FIG. 3) because of a member in the form of a disc 28 which is
attached to a hub 30 and mounted on the shaft 22 for rotation with
the impeller 16. The disc 28 is co-axial with the impeller 16, but
is of a diameter less than a diameter of the impeller measured
across the circles circumscribed by the tips 32 of the blades 18.
The discharge flow from the impeller is increased in velocity
because of the confinement of the flow by the disc 28, and
continues generally axially toward the bottom 12 of the tank where
the flow turns, and the flow is axially upward. Baffles 34,
constrain the upward flow to be generally in the axial direction.
The flow then turns downwardly, to provide an inlet flow into the
upper or suction side of the impeller 16. The impeller 16 may be
any conventional type of axial flow impeller. A suitable impeller
is the high efficiency type A310 which is available from
Lightnin.RTM. Mixers of Rochester, N.Y. 14611 USA. Further
information about the A310 impeller may be found in U.S. Pat. No.
4,468,130 issued to R. J. Weetman.
A sparge 38 is disposed below the impeller 16 in the outlet flow
which is generally axially downward from the tip region of the
impeller. The sparge releases gas radially outward of the periphery
of the disc 28, but inwardly from the circular path described by
the rotation of the tips 32. The sparge 38 may be a ring sparge
having a circular tube into which gas is blown from the outside of
the tank via a pipe 40. The tube providing the sparge ring 38 may
be mounted on legs 42 extending from the bottom of the tank. The
sparge ring 38 has a plurality of openings 44 which are in portions
of the wall of the sparged tube radially most distance from the
center of the sparge ring 38. The ring is desirably co-axial with
the shaft 22.
It has been discovered, in accordance with the invention, that
collection of gas on the impeller blades 18 and the consequent
flooding of the impeller is inhibited when the distance between the
surface 50 of the disc, which faces the impeller and is in the path
of the discharge flow therefrom, is within a certain spacing or
distance from the impeller. This distance may be measured between
the midline 52 of the impeller and the surface 50 in the direction
axially of the shaft 22. The spacing depends upon the density and
viscosity of the liquid in the tank, including any materials
suspended in the liquid. When the liquid is water the distance
(indicated as S in FIG. 3) is preferably about 7% of the diameter
of the impeller. By the diameter of the impeller is meant, the
diameter of the circle described by the tips 32 as the impeller
rotates. The (S) may vary from 20% to 5% of the impeller diameter,
depending upon the characteristics of the liquid being circulated.
In other words the spacing is of the order of 10% of the impeller
diameter. In any event the spacing is selected to be sufficient to
create the radial flow which sweeps across the impeller, removing
with the flow, gas which tends to collect on the surfaces of the
impeller blades. In effect. the flow creates a pressure
differential which decreases in the direction radially outwardly of
the blades and promotes the movement of the gas away from the
blades. The flow is of maximum velocity and is generally axial near
the tips. In order not to interfere with this flow the diameter of
the disc 28 is less than the diameter of the impeller. Preferably
the diameter of the disc is about 75% of the diameter of the
impeller, but can range from 65% to 85% of the diameter of the
impeller 26. The gas leaving the sparge tube 38 encounters this
high velocity flow and is sheared into bubbles. The bubbles
continue towards the bottom of the tank and circulate across the
bottom of the tank and then upwardly towards the top of the liquid
in the tank. During the circulation the mass of the gas is
transferred to the liquid.
Referring to FIG. 7 there is shown two curves one in solid lines
and the other in dash lines. The curve in dash lines represents a
system such as shown in FIGS. 1 through 3, or in FIGS. 4 through 6,
but without the disc 28. The X axis of the curve is calibrated in
aeration number Na which is function of the volume of air sparged
into the tank (the gas rate). Na is related to N D.sup.3 Nq where N
is the speed of the impeller, D is its diameter and Nq is the flow
number. The curves illustrate that the K factor remains essentially
constant with increasing aeration number or gas flow rate when the
mixing system is equipped with the disc in the above described
relationship to the impeller.
FIGS. 1 and 2 illustrate the case where the impeller is spaced from
the bottom of the tank by a distance C as measured between the
midline of the impeller and the bottom of 12 of the tank. This
distance C is preferably about 2/3 of the impeller diameter. C is
shown as d in FIG. 1. C is preferably in the range of one impeller
diameter or less where the ratio of the diameter of the impeller to
the diameter of the tank is approximately 1/3.
Referring to FIGS. 4, 5, and 6 there shown a mixing system which is
similar to the system shown in FIGS. 1, 2, and 3, and like parts
are identified by the same referenced numerals. The impeller 27
shown is the type A315 which is a four bladed impeller also
available from Lightnin Mixers of 135 Mount Read Blvd. Rochester,
N.Y. 14611 USA. The A315 impeller is described in Weetman and Howk
U.S. Pat. No. 4,896,971 issued Jan. 30, 1990.
The disc 29 is similar in its spacing from the impeller and its
diameter relative to the diameter of the impeller 27, as in the
case of the disc 28 described above and shown in FIGS. 5 1, 2, and
3. The disc 29 is part of a sparge 31. It is spaced from another
disc 33 which is of the same diameter as the disc 29. The disc 33
has a central opening 35 and sits on a stub pipe 37 into which gas
to be sparged is blown via a pipe 39. The discs are assembled on
and spaced apart by an apertured collar 41. The annular space
between the discs 29 and 30 directs the gas to flow outwardly from
around the periphery of the space between the discs 29 and 31, into
the generally downward and axial flow from the tip region of the
impeller 27. The gas is then sheared into bubbles and circulated by
the impeller 27 radially towards the side walls of the tank 10 and
then axially downwardly as inlet flow to the down pumping impeller
27. The direction of the circulation is shown by the arrows 45.
From the foregoing description it will apparent that there has been
provided an improved gas dispersion and mass transfer mixing system
wherein flooding is inhibited and gas flow rates without flooding
may be two to six times greater than obtainable without the
improvements provided by the invention. Variations and
modifications, within the scope of the invention, will undoubtedly
suggest themselves to those skilled in the art. Accordingly, the
foregoing description should be taken as illustrative and not in a
limiting sense.
* * * * *